The invention relates to a low-pressure gas discharge lamp comprising a gas discharge vessel with a gas filling, and comprising electrodes and means for generating and maintaining a low-pressure gas discharge.
Light generation in low-pressure gas discharge lamps is based on the principle that charge carriers, particularly electrons but also ions, are accelerated so strongly by an electric field between the electrodes of the lamp that collisions with the gas atoms or molecules in the gas filling of the lamp cause these gas atoms or molecules to be excited or ionized. When the atoms or molecules of the gas filling return to the ground state, a more or less substantial part of the excitation energy is converted to radiation.
Conventional low-pressure gas discharge lamps comprise mercury in the gas filling and, in addition, are equipped with a phosphor coating on the inside of the gas discharge vessel. A drawback of the mercury low-pressure gas discharge lamps resides in that mercury vapor primarily emits radiation in the high-energy, yet invisible UV-C range of the electromagnetic spectrum, which radiation must first be converted by the phosphors to visible radiation having a much lower energy level. In this process, the energy difference is converted to undesirable thermal radiation.
In addition, the mercury in the gas filling is being regarded more and more as an environmentally harmful and toxic substance that should be avoided as much as possible in present-day mass products as its use, production and disposal pose a threat to the environment.
It is known already that the spectrum of low-pressure gas discharge lamps can be influenced by substituting the mercury in the gas filling with other substances.
For example, GB 2 014 358 A discloses a low-pressure gas discharge lamp comprising a discharge vessel, electrodes and a filling that contains at least one copper halogenide as the UV emitter. This copper halogenide-containing low-pressure gas discharge lamp emits in the visible range as well as in the UV range around 324.75 and 327.4 nm.
It is an object of the invention to provide a low-pressure gas discharge lamp the radiation of which is as close as possible to the visible range of the electromagnetic spectrum.
In accordance with the invention, this object is achieved by a low-pressure gas discharge lamp comprising a gas discharge vessel containing a gas filling with a chalcogenide of the elements of the 4th main group of the periodic system of elements and a buffer gas, and comprising inner or outer electrodes and means for generating and maintaining a low-pressure gas discharge.
In the lamp in accordance with the invention, a molecular gas discharge takes place at a low pressure, which gas discharge emits radiation in the visible and near UVA range of the electromagnetic spectrum. As this radiation originates from a molecular discharge, the type of chalcogenide, possible further additives as well as the internal pressure of the lamp and the operating temperature enable the exact position of the continuous spectrum to be controlled.
In combination with phosphors, the lamp in accordance with the invention has a visual efficiency which is substantially higher than that of conventional low-pressure mercury discharge lamps. The visual efficiency, expressed in lumen/Watt, is the ratio between the brightness of the radiation in a specific visible wavelength range and the energy for generating the radiation. The high visual efficiency of the lamp in accordance with the invention means that a specific quantity of light is obtained at a smaller power consumption.
The chalcogenides of the elements of the 4th main group of the periodic system of elements, for example silicon, germanium, tin and lead, have a high dissociation energy. As a result, during the gas discharge only a small proportion of the molecules in the gas phase is split by electron impact ionization, and only few chalcogenide ions occur during the gas discharge. This also has a favorable effect on the visual efficiency of the lamp.
In addition, the use of mercury is avoided.
As an UV-A lamp, the lamp in accordance with the invention is advantageously used for sunbeds, and as a disinfecting lamp and a lacquer-curing lamp. For general illumination purposes, the lamp is combined with appropriate phosphors. As the losses caused by Stokes"" displacement are small, visible light having a high luminous efficiency above 100 lumen/Watt is obtained.
Within the scope of the invention it may be preferred that the chalcogenide is selected among the group consisting of the sulphides, selenides and tellurides.
Within the scope of the invention, it may be preferred that the element of the 4th main group of the periodic system of elements is selected among the group consisting of silicon, germanium, tin and lead.
It is particularly preferred that the chalcogenide is selected among the group consisting of SiS, GeS, GeSe, GeTe, SnS, SnSe and SnTe.
Particularly advantageous effects in relation to the state of the art are achieved if the gas filling comprises germaniumselenide GeSe. In this case, a gas discharge with a wide continuous spectrum is obtained.
It may alternatively be preferred that the gas filling comprises germaniumsulphide GeS. A gas filling comprising germaniumsulphide is characterized by a high vapor pressure.
A further improved efficiency is achieved if the gas filling comprises a mixture of two or more chalcogenides of silicon, germanium, tin and lead.
It is preferred that in the chalcogenide, the molar ratio n between the chalcogen and the element of the 4th main group of the periodic system of elements is 0.8xe2x89xa6nxe2x89xa61.2.
As the buffer gas, the gas filling may contain an inert gas selected among the group consisting of helium, neon, argon, krypton and xenon.
Within the scope of the invention it may be preferred that the gas discharge vessel comprises a phosphor coating on the outside surface. The UVA radiation emitted by the low-pressure gas discharge lamp in accordance with the invention is not absorbed by the customary glass types, but goes through the walls of the discharge vessel substantially free of losses. Therefore, the phosphor coating can be provided on the outside of the gas discharge vessel. This results in a simplification of the manufacturing process.
It may also be preferred that the gas discharge vessel comprises a phosphor coating on the inner surface.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiment(s) described hereinafter.